The present invention generally relates to electronic circuits, and more particularly to an electronic circuit which is especially suited for use with an input buffer circuit of an electronic device which carries out a digital signal processing in an automobile for example.
Recently, electronic devices control various parts of an automobile. For example, signals from various sensors are supplied to an electronic device of the automobile such as a control unit wherein a digital signal processing is carried out. In this case, there is a demand for the control unit to accurately read the signals from the various sensors and operating states of other processing circuits.
FIG. 1 shows an example of a conventional electronic circuit which is used in an automobile. A processing circuit 17 enters an output signal of a sensor 16. For example, the processing circuit 17 is an air conditioning system which drives a coil L1. An output signal of the processing circuit 17 is applied to a terminal 10 and is supplied to the coil L1 via transistors Q1 and Q2 which form a Darlington pair and a protection resistor R1. A control unit 11 controls a fuel injection quantity and the like of an engine (not shown). The control unit 11 has a terminal 12 which receives a voltage indicative of a driving state of the coil L1. The voltage received from the terminal 12 is supplied to a non-inverting input terminal of a comparator 13, and the control unit 11 carries out a control such as increasing an idling speed of the engine depending on an output signal of the comparator 13. The terminal 12 is supplied with a power source voltage +B from a battery 14 via the coil L1. For example, the power source voltage +B is +12 V.
The control unit 11 has the form of a semiconductor integrated circuit package, and is supplied with a power source voltage Vcc from the battery 14 via a regulator 18. For example, the power source voltage Vcc is +5 V. A reference voltage Vref which is obtained by dividing the power source voltage Vcc by a voltage divider made up of resistors R2 and R3 is supplied to an inverting input terminal of the comparator 13. The power source voltage +B from the battery 14 is also supplied to loads such as a light 15.
The reference voltage Vref is obtained by dividing the power source voltage Vcc. For this reason, when the power source voltage Vcc deviates, the reference voltage Vref undergoes a corresponding deviation and the deviation of the power source voltage Vcc is compensated thereby.
On the other hand, the signal voltage which is supplied to the terminal 12 is dependent on the power source voltage +B but a deviation of the power source voltage +B is unrelated to a deviation of the power source voltage Vcc as long as the deviation of the power source voltage +B is within a deviation range which can be compensated by the regulator 18. As a result, even when no signal change occurs at the terminal 10, there is a problem in that the control unit 11 may carry out an erroneous operation due to the deviation of the power source voltage +B.
In addition, the ground of the electronic circuit shown in FIG. 1 is a metal body (not shown) of the automobile and includes some resistance component. For this reason, when a switch SW closes and a large current flows through the load such as the light 15, an emitter potential of the transistor Q2 may rise. This potential rise corresponds to the deviation of the power source voltage +B and causes a problem similar to the above by changing the signal voltage at the terminal 12.